In this proposal two classes of mutants are to be studied. On the one hand we expect to use mutants in fatty acid biosynthesis to explore their role in the genesis of mitochondrial membranes in the yeast Saccharomyces cerevisiae. Particular attention will be paid to the synthesis and assembly of the components of cytochrome c oxidase. The one mutant now being intensively studied is KD 115, an unsaturated fatty acid auxotroph, which when grown on trans unsaturated fatty acid respires poorly, has low cytochrome oxidase activity and low haem a and haem b content compared to the same cells grown on cis unsaturated fatty acid supplements. The oligomycin sensitive ATPase and the liquid composition of the different cultures is comparable. Other conditional mutants of phospholipid biosynthesis are to be sought by a variety of techniques with the object of using these for a further exploration of the role of lipids in mitochondrial biosynthesis. Particular attention is to be paid to acidic phospholipid biosynthesis and to this end, we have partially purified (600 fold) and studied the kinetics of the enzyme CTP phosphatidic acid cytidyl transferase, which is predominantly located in the inner mitochondrial membrane.